Yes it does seem a bit strange that Marauder has a stronger defense than DPS Juggernaut.
I'd say that a Marauder has better defensive cooldowns than *tank* Juggernaut. Cloak of Pain uptime to mitigation is much better than Invincible. Undying Rage is better than Endure Pain in almost every way imaginable. Not even taking into account Obfuscate which does not affect Ops bosses and Camo because it drops aggro (but mitigates 50% of a heavy nontargeted attack).
As for the original post, yeah. Heavy armour is overrated. Realistically, raw armour bonus is about 5% between heavy and medium.